Clamping camera to path, existing keyframe positions are thrown out when path is extended?
Maybe there's another solution but here is a trick that may satisfy you:


Create your curve, give it the right direction, put its origin at the beginning.





Create an object (here a sphere), in Edit mode select all the vertices and assign them a vertex group, in Object mode put the object at the exact same beginning point as the curve, give it a Curve modifier with the curve as Object.





Create your camera, give it a Copy Location constraint with the sphere as Target, and the sphere's vertex group as Vertex Group. 





Now when you move your sphere forwards on the right axis (here the Y axis), it follows the curve and the camera follows as well. If you extrude the curve, it won't move the camera's position.